Chemical vapor deposition (CVD) is a widely used method for growing materials on surfaces within a gaseous environment. CVD involves exposing some solid surface (substrate) to an environment consisting of gases which react with the substrate to form a desired layer of new material composed in part of the elements present in the gases. In this manner, metals may be deposited on semiconductors, polymeric materials may be deposited on metals, and many other practical coatings or overlayers may be so produced. Often it is also necessary to supply energy in the form of heat or plasma excitation in order to assist the deposition process.
While CVD is widely understood and employed in a myriad of useful applications, it is difficult or impossible to grow some materials by conventional CVD techniques if their formation requires gases that are not compatible with each other. For example, it may not be possible to produce a material which requires both hydrogen and oxygen in the formation process because the hydrogen may combine with the oxygen in the gas so rapidly that the desired reaction may be inhibited.
Also, if forming a layer of a specific material requires several chemical reactions in a specific sequence, a single gas environment may result in undesirable out-of-sequence chemical reactions. One immediately apparent solution to this problem would be to transport the substrate from one CVD chamber to another in order to produce the desired sequence of reactions or to eliminate the undesired competitive reactions. However, unless this can be done very rapidly, unwanted reactions or other changes in the surface may occur during the transit time, and the overall time required to grow the desired thickness of material will be prohibitively long. Similarly, sequentially introducing different gases into a single chamber requires either an unacceptable amount of time or an imperfect exchange, so one gas environment contaminates the next.
U.S. Pat. No. 4,664,743 to Moss et al. describes a sequential film growing method by proposing to transport the substrate between a plurality of physically separated environments, each of which is needed to grow a particular material. With the Moss et al. approach, it is possible to produce layers of different materials by moving the substrate from one environment to the other to enable sequential exposure to at least two gas flows. However, the Moss et al. approach is limited to growing only those materials which are capable of being grown in a single gas environment. Also, Moss et al. make no provision for excitation of the different gas environments.
U.S. Pat. No. 4,622,919 to Suzuki et al. teaches film growth by sequential vapor deposition, not CVD. While Suzuki teaches plural gas environments within a reaction vessel, it is not understood how the Suzuki et al. devices would be able to keep the different gas environments separate within the reactor, and would require very high vacuum conditions in which the atoms suffered few collisions between the source and the substrate.
While diamond films may be grown by conventional CVD techniques using mixtures of hydrogen and methane in an electrical discharge, these conventional techniques have required that the substrate be heated to approximately 800.degree. C. This temperature is too hot to provide useful coatings on most materials because of the resultant thermal damage to the substrate. Thus, one hitherto unsolved need has been for a sequential CVD process for growing diamond coatings at lower process temperatures.